Assessing a parameter of cells in the batteries of uninterruptable power supplies

ABSTRACT

A method and apparatus for measuring the electrical efficacy of one or more battery cells for use in an uninterruptable power supply are disclosed. The efficacy is determined by making use of the ripple current which flows in the battery cells when in use in the uninterruptable power supply. Simultaneous measurement, for example, of the ripple current and a corresponding voltage component enables the internal impedance of a battery cell to be determined, the impedance acting as an indicator of electrical efficacy.

The present invention relates to uninterruptable power supplies and inparticular to a method and apparatus for assessing a parameter of thecells in the batteries of such power supplies.

Uninterruptable power supply systems are used in situations whereunexpected loss of power is particularly undesirable, for example byfinancial institutions, telecommunications installations, the utilities,hospitals and the military. They are needed where loss of power isunacceptable, for example where hospital patients rely on life supportsystems, or where data loss due to a computer shut down would beunacceptable as in a financial institution. The battery of theuninterruptable power supply is typically the last line of defenceagainst total shutdown during power outages.

A typical arrangement of an uninterruptable power supply is shown inFIG. 1. An external alternating current (a.c.) power supply 1, generallysupplied by an outside utility company, is converted to direct current(d.c.) by a rectifier 2. The rectified d.c. is converted back to a.c. byan inverter 3 for supply to the power supply user 4. A battery 5 isconnected to the d.c. part of the system in such a way that the chargeon the battery is maintained during normal operation of the powersupply. The battery may typically comprise a large number of lead acidcells. Should the external power supply 1 fail for any reason, thebattery 5 maintains the operating voltage of the d.c. part of the systemso that the power supply to the user 4 is maintained.

Battery 5 is shown, for convenience, and simplicity connected betweenthe D.C. voltage and earth. In practise, however, especially on largerbatteries, the battery is generally at a voltage floating with respectto earth, as supplied by the rectifiers.

Unless the battery is healthy, it may not be able to carry the requiredelectrical load when the a.c. supply is cut off. Thus it is desirable tobe able to determine an indication of the condition of the cells in thebattery, so as to be able to take further action, for example byrepairing or replacing a cell, if a cell is unlikely to be able to meetthe power demands whilst the a.c. supply is cut off.

Batteries are generally manufactured with a certain life span that isdependent on environmental criteria and the number of dischargessupplied by the battery. Some of the discharges will be due to use ofthe battery during a.c. power cuts, but some may occur during loadtesting. One typical method of determining battery health employs a loadtest. During this load test, the battery is disconnected from the powersupply system and discharged across a load such as a resistor bank. Therate at which the cell voltage then decays is indicative of thebattery's health and ability to sustain the power supply should the a.c.supply be cut. Weak battery cells display earlier and more rapid signsof voltage decay. The voltage decay characteristic obtained during aload test correlates well with the expected performance, but the test islabour intensive and cannot easily be performed with the batteryconnected to the operating uninterruptable power supply. Furthermore,battery lifetime is reduced as a result of the required discharge.

To prolong battery lifetime, therefore, modes of testing that do notinvolve large discharges have been developed. For example, reducing thedepth of discharge during battery testing greatly improves batterylifetime. An alternative to load testing is to use impedancemeasurements to determine battery condition.

Any device through which an electrical current will flow exhibits animpedance to that flow. In a lead acid battery the impedance comprisespure resistance components such as the battery terminals, plates, andthe resistance of the electrochemical path, and capacitative components,in particular of the parallel plates. The impedance of the battery willtherefore depend on the frequency at which it is measured. Detailedanalysis of battery impedance measurements is difficult, requiringcomplex calculations. No universal equivalent circuit is available todescribe the response of even a single electrode. The situation is yetmore complicated when considering complete cells or batteries, with theinfluence on impedance of all the individual components being difficultor impossible to separate. For this reason, battery impedancemeasurements in practice are usually limited to one or a few impedancemeasurements at fixed frequencies. Deviations of a single cell from anorm may then indicate that this cell is faulty.

Although the battery resistance can be measured using a d.c dischargeacross two or more different loads, battery lifetime may be affected bythe significant discharge required to obtain repeatable readings, and along measurement cycle is needed to ensure that battery recovers beforetaking measurements from the next cell. These problems do not occurduring an a.c. impedance measurement. A variety of frequencies have beensuggested or used for such measurements, ranging from 10 Hz to 1 mHz. Asignal generator is used to apply an a.c. signal of the requiredfrequency to individual cells or to the whole battery. Current andvoltage readings are then made to determine the impedance of individualcells.

By use of an on-line monitor it is possible to look for changing cellfloat voltages and cell impedance values that signal that thecharacteristics of the cell are changing. In such an application it doesnot matter that a physically correct value of impedance is not returnedby the monitor. Rather, the monitor needs to determine whether a measureof the impedance of a given cell or group of cells has changedsignificantly over time, perhaps with respect to some baseline or norm,or whether the measure of impedance of one cell or group of cells issignificantly different from the battery average. Long term stability isan important indicator of cell performance and health. The skilled andexperienced person is able to make a decision to make further checks ona cell or group of cells, perform repairs or install a replacement,based on the measurements of impedance. Automatic monitoring equipment,perhaps embodied in a computer connected to a telecommunications link,may be used to assist in this process.

Impedance monitors of the prior art have measured cell impedance byinjecting an a.c. signal of a given frequency into the battery and byfiltering measurements of voltage and current at the same frequency.Because of the large capacity of the batteries used in uninterruptablepower supplies the signal generator may need to be of a considerablesize.

It is an object of the present invention to provide an improved methodfor determining the condition of cells within the batteries ofuninterruptable power supplies.

According co the present invention, there is provided a method ofmeasuring the electrical efficacy of one or more battery cells for usein an uninterruptable power supply, the method comprising: measuring atleast one of an a.c. component of a current through the battery cell orcells and an a.c. component of a voltage across the said battery cell orcells, the a.c. component arising from a ripple current in the saidbattery cell or cells in use; and determining the electrical efficacy ofthe cell or cells on the basis of the or at least one of the measureda.c. current and voltage components.

The ripple current in the battery results from the normal operation ofthe uninterruptable power supply. In particular, it may result from theoperation of those components converting between direct and alternatingcurrent. Electrical signals related to the ripple current include theripple current itself and ripple voltages driving or driven by theripple current.

Preferably, the step of determining the electrical efficacy includesobtaining a numerical value from the, or at least one of the, measureda.c. current and voltage components.

In that case, the electrical efficacy of the or each battery cell may bedetermined by comparison of the said numerical value with acorresponding further numerical value obtained by measurement of a.c.current and/or voltage components from one or more different cells. Theelectrical efficacy may in a particularly preferred embodiment bedetermined by comparison of the said numerical value with the average ofa plurality of further numerical values obtained by measurement of a.c.current and/or voltage components from a corresponding plurality ofseparate arrays of single or multiple cells respectively.

Alternatively, the electrical efficacy of the battery cell or cells maybe determined by comparison of the said numerical value with acorresponding predetermined numerical value.

The method may further comprise the steps of measuring both the a.c.component of current through the battery cell or cells and the a.c.component of the voltage across the battery cell or cells; and obtaininga value for the internal impedance of the battery cell or cells via acombination of the said current component and the said voltagecomponent.

Although the impedance of the cell or cells is measured in preference,other parameters may be conveniently assessed, as will be apparent tothe person skilled in the art. For example the resistive, capacitativeor inductive components of the battery impedance, or the powerdissipated in the cell or group of cells within a given frequency bandmay be determined. These and other parameters may be of use in assessingthe condition of the cell or group of cells.

Preferably, the step of measuring at least one of the a.c. components ofa current and a voltage includes the steps of: measuring electricalsignals representative of at least one of the voltage level across thecell or cells and the current level through the cell or cells; andfrequency filtering the or each measured electrical signal to extractthe said a.c. component arising from the ripple current. In that case,the step of filtering includes isolating a band of frequencies from theor each said electrical signals.

The band of frequencies may include at least one harmonic frequency ofthe a.c. mains frequency, such as harmonics of 50 Hz or 60 Hz. Mostpreferably, components at 900 Hz and 1080 Hz are chosen.

The invention also extends to an apparatus for measuring the electricalefficacy of one or more battery cells for use in an uninterruptablepower supply, the apparatus comprising an ammeter arranged to measure ana.c. component of a current through the battery cell or cells, the a.c.current component arising from a ripple current in the said battery cellor cells in use, the electrical efficacy of the cell or cells beingdetermined on the basis of the measured a.c. current component.

In yet a further aspect, the invention resides in an apparatus formeasuring the electrical efficacy of one or more battery cells for usein an uninterruptable power supply, the apparatus comprising a voltmeterarranged to measure an a.c. component of a voltage across the batterycell or cells, the a.c. voltage component arising from a ripple currentin the said battery cell or cells in use, the electrical efficacy of thecell or cells being determined on the basis of the measured a.c. voltagecomponent.

In that case, the apparatus may further comprise an ammeter arranged tomeasure an a.c. component of a current flowing through the battery cellor cells, the a.c. current component also arising from the said ripplecurrent in the said battery cell or cells in use, the electricalefficacy of the cell or cells being determined on the basis of both themeasured a.c. voltage component and the a.c. current component.

A filter such as a fifth order band pass filter may also be provided toisolate harmonic frequencies of mains frequencies, for example.

Advantageously, apparatus according to the present invention may becharacterised in that it does not comprise a signal generator forinjecting a current into the battery.

Embodiments of the present invention will now be described by way ofexample only and with reference to the drawings, of which:

FIG. 1 shows a typical arrangement of an uninterruptable power supply;

FIG. 2 is a schematic diagram showing apparatus for measuring theimpedance of cells or groups of cells in a battery for anuninterruptable power supply; and

FIG. 3 is a schematic diagram showing apparatus embodying the presentinvention, for measuring the voltage across cells or groups of cells ofa battery of an uninterruptable power supply.

Alternating current methods of measuring impedance in battery cells havehitherto required the injection of an alternating current into thebattery string. The method of the present invention utilises electricalsignals related to the inherent ripple current present in almost alluninterruptable power supply batteries.

Ripple current is caused by the power supply utilising the power storagecapacity of the battery. The battery is used to provide current for theinverter to produce an a.c. current from the d.c. bus voltage. Thisripple current typically has a waveform that repeats at least at thefrequency of the alternating current supplied by the inverter, but alsocontains many higher order harmonics and noise. The peak to peakmagnitude of the ripple current is typically 20% to 100% of the actualload current supplied by the inverter. The ripple current is associatedwith related electrical signals, in particular, ripple voltages acrossvarious battery components.

One embodiment of the invention will now be described with reference toFIG. 2 which shows, schematically, an apparatus for measuring theimpedance of cells or groups of cells in a battery for anuninterruptable power supply.

As seen in FIG. 2, a number of cells 11 are together constituting abattery 30 connected in series across a d.c. bus 10, 12 of anuninterruptable power supply. The cells 11 make up the battery used tomaintain the voltage of the d.c. bus during a period when the a.c.supply 1 (FIG. 1) is cut. The cells are typically supplied grouped intounits which are then installed in the uninterruptable power supply. Awhole unit may then be replaced if found to be faulty.

An ammeter 13 is connected in series with the cells 11 and measures theinstantaneous current flowing through the battery at a selectedfrequency.

A voltmeter 14 is likewise connected in parallel with the cells 11 andmeasures the instantaneous voltages across each of the cells 11, againat the selected frequency. The apparatus may conveniently be protectedby individual current limiting protection resistors and the voltagesmeasured using a number of divider resistor networks, yielding voltagesignals from the junction between each cell. These features will bedescribed in more detail with reference to FIG. 3 below.

At any one time, the current measurement or the voltage measurement of aparticular cell is selected by a multiplexer array 15, under the controlof a central processing unit (CPU) 20. Typically the multiplexer array15 may select the voltage measurement signals from both sides of a cell.As will be described in connection with FIG. 3, these signals may thenbe buffered before a differentiator circuit removes the d.c. componentsand a subtractor circuit converts the differential of the two signals toa single bipolar signal.

The current or voltage signal selected by the multiplexer array isfiltered using a high order band pass filter 16 arranged to pass anarrow band of frequencies around a selected frequency. The resultingfiltered signal is converted to a d.c. voltage using a root-mean-squarea.c. to d.c. converter 17 and is sampled using an analogue to digitalconverter 18. The resulting digital data is read by the CPU 20, which islinked to each of the multiplexer array 15, band pass filter 16, a.c. tod.c. converter 17 and analogue-to-digital converter 18 by a control line19.

The high order band pass filter may be centered on a frequency that isselected to best indicate battery condition. A frequency of 900 Hz maybe chosen in the United Kingdom and Europe where the frequency of theexternal a.c. supply is 50 Hz. Similarly, in the United States ofAmerica a central filter frequency of 1080 Hz may be chosen for use with60 Hz mains a.c.

Measurements of battery impedance at various other frequencies, however,have been found to yield good indications of battery condition, and anysuitable frequency may be used. A clock sweepable 5th order band passfilter under the control of a programmable clock filter has been foundto provide good results in the operation of the present apparatus, andallows the central frequency of the filter to be varied conveniently ifrequired. Other filter arrangements may be equally suitable.

The CPU 20 calculates the ratios of the digitised voltage and currentdata to yield measurements of the impedance of the battery cells. Bymeans of the multiplexer, the CPU 20 is able to sequentially select thevoltage signal of each cell 11. The CPU may record the measurements ofimpedance, preferably in a non-volatile memory and/or on a mass storagedevice such as a magnetic device (not shown).

It is not necessary for calibrated measurements of impedance to beobtained, since the conditions of the cells of the battery can beassessed by monitoring changes in impedance over time, or by comparingthe simultaneous impedances of a cell to other cells or the batteryaverage.

By means of a telecommunications link 21, the CPU 20 may be remotelyinterrogated to obtain the impedance measurements. An assessment of theconditions in the battery may then be made. The CPU 20 may convenientlybe programmed to monitor the impedance measurements to raise an alarmvia the telecommunications link 21 if any significant changes inimpedance occur, such as changes of the impedance of a single cell thatexceed a preset threshold, or the impedance of a single cell differingfrom the battery average by more than a preset threshold.

The apparatus shown may be used to measure the impedance of cells of thebattery. By monitoring how the measurement of impedance of individualcells or groups of cells changes over time, the skilled person is ableto obtain an indication of the conditions of individual cells, forexample on observing an increase in the impedance of a single cell thatis characteristic of a fault in or problem with that cell.

Similar apparatus may be used to monitor other parameters of individualcells or groups of cells. The power dissipated by a cell within a givenfrequency band, or the resistive, capacitative or inductive componentsof the impedance may, for example, be useful for obtaining an indicationof the conditions of individual cells, and may be monitored usingvariations to the described apparatus that will be familiar to theperson skilled in the art.

FIG. 3 shows, schematically an apparatus for measuring the voltageacross cells or groups of cells within the battery of an uninterruptablepower supply. The diagram illustrates a particular configuration ofapparatus that may be used to implement the voltage measurement functionof the apparatus shown in FIG. 2. Accordingly, features common to FIGS.2 and 3 are labelled with like reference numerals.

The battery 30 comprises a number of cells or groups of cells 11.Electrical connections from the junctions between each cell or group ofcells 11 to a voltage measuring apparatus are made through currentlimiting protection resistors 31 and through pickup wires 32.

The current limiting protection resistors 31 protect the voltagemeasuring apparatus from current surges and electrical damage.

The pickup wires 32 connect the current limiting protection resistors 31to an array of divider resistor networks 33 configured to scale thevoltages being measured to a level appropriate for the electroniccircuits that follow.

A multiplexer array 15 samples each pair of adjacent voltagemeasurements in turn. The pair of voltage outputs from the multiplexerarray 15 passes through buffers 35 and to a differentiator 36 whichremoves the d.c. component of the signal. The resultant pair of voltageoutputs is then passed to a subtractor 37 which converts thedifferential voltage input signal to a single bipolar output signal. Thesingle voltage output from the subtractor 37 is then filtered by a clocksweepable fifth order band pass filter 16. The central frequency of theband pass filter is controlled by a programmable clock 39. A centralfrequency for the filter may be chosen to best indicate batterycondition as described above.

The filtered signal is passed through a root-mean-square (r.m.s.) to d.cconverter 17 to convert the filtered alternating signal to a d.c.voltage which is sampled by an analogue to digital converter 18. Adigital output 42 from the analogue-to-digital converter may be passedto a central processing unit as described in connection with FIG. 2.

An equivalent apparatus may be provided for measuring the currentpassing through the battery, employing a similar filter, root meansquare to d.c. converter and analogue to digital converter.Alternatively, an ammeter or other circuitry may be provided between thebattery 30 and the multiplexer array 15 to enable the apparatus tosample the ripple current as well. In either case, the digital outputthat represents the measured current is passed to the central processingunit for the purposes of calculating the impedance of each cell or groupof cells.

Typically, batteries for uninterruptable power supplies comprisemultiple cells grouped into units. It will be understood that theapparatus described herein is equally suitable for measuring individualcells, groups of cells or indeed the whole battery, as desirable. Itwill also be appreciated that the specific implementation of the variousfeatures shown in FIG. 3 is a matter of design choice. For example,discrete analogue or digital components might be used, or integratedcircuits as appropriate. Alternatively, software filtering and softwarecalculation of the root-mean-square to d.c. function could be employed.Likewise, common components may be used to process both the voltage andcurrent components, or separate circuits could be used.

If only a very small amount of ripple current flows through the batteryof an uninterruptable power supply system, and this current isinsufficient for the effective operation of the impedance measurementequipment, then a signal generator may be provided to inject currentinto the system during times of impedance measurement.

What is claimed is:
 1. A method of measuring the electrical efficacy ofa plurality of groups of one or more battery cells while in use in anuninterruptable power supply, the method comprising: measuring, usingsimultaneous electrical connections across each group of cells, an a.c.component of a voltage across each group of cells, the a.c. componentarising from a ripple current in said plurality of groups of one or morecells while in use; and determining the electrical efficacy of eachgroup of cells on the basis of the measured a.c. voltage components. 2.The method of claim 1, in which the step of determining the electricalefficacy of each group of cells includes obtaining numerical values fromthe measured a.c. voltage components.
 3. The method of claim 2, in whichthe electrical efficacy of one of the groups of battery cells isdetermined by comparison of the associated numerical value with acorresponding further numerical value obtained by measurement of an a.c.voltage components from another of the other groups of cells.
 4. Themethod of claim 3, in which the electrical efficacy of one of the groupsof battery cells is determined by comparison of the associated numericalvalue with the average of a plurality of further numerical valuesobtained by measurements of the a.c. voltage components from acorresponding plurality of the other groups of cells.
 5. The method ofclaim 2, in which the electrical efficacy of each group of battery cellsis determined by comparison of each associated numerical value with acorresponding predetermined numerical value.
 6. The method of claim 1,further comprising the steps of measuring both an a.c. component ofcurrent through the plurality of groups of battery cells and the a.c.component of the voltage across each said group of battery cells; andobtaining a value for the internal impedance of each group of batterycells via a combination of said current component and said voltagecomponent.
 7. The method of claim 1, in which the step of measuringincludes the steps of: measuring electrical signals representative ofone of the voltage level across each group of cells and the currentlevel through the plurality of groups of cells; and frequency filteringthe each-measured electrical signal to extract the measured a.c.component arising from the ripple current.
 8. The method of claim 7, inwhich the steps of filtering includes isolating a band of frequenciesfrom each measured electrical signals.
 9. The method of claim 7, inwhich the step of filtering includes isolating a band of frequenciesincluding those frequencies which include at least one harmonicfrequency of an a.c. main frequency.
 10. The method of claim 9,including isolating a band of frequencies around 900 Hz.
 11. The methodof claim 9, including isolating a band of frequencies around 1080 Hz.12. An apparatus for measuring the electrical efficacy of a plurality ofgroups of one or more battery cells while in use in an uninterruptablepower supply, the apparatus comprising: simultaneous electricalconnections across each group of cells; voltage measuring meansconnected to said simultaneous electrical connections and arranged tothereby measure an a.c. component of a voltage across each group ofcells, the a.c. voltage component arising from a ripple current in saidplurality of groups of one or more battery cells while in use, theelectrical efficacy of each group of cells being determined on the basisof the measured a.c. voltage components.
 13. An apparatus as claimed inclaim 12, further comprising an ammeter arranged to measure an a.c.component of a current flowing through the battery cell or cells, thea.c. current component also arising from said ripple current in saidplurality of groups of one or more battery cells while in use, theelectrical efficacy of each group of cells being determined on the basisof both the measured a.c. voltage components and the a.c. currentcomponent.
 14. An apparatus as claimed in claim 13, in which theelectrical efficacy is determined on the basis of impedances calculatedfrom the ratio of said measured a.c. voltage components to said measureda.c. current component.
 15. An apparatus as claimed in claim 12, furthercomprising a frequency filter arranged to pass a band of frequenciesincluding the frequency of the a.c. component arising from the ripplecurrent.
 16. An apparatus as claimed in claim 15 in which the filter isa fifth order band pass filter.
 17. An apparatus as claimed in claim 16,in which the filter is arranged to pass a band of frequencies includingat least one harmonic frequency of an a.c. main frequency.
 18. Anapparatus as claimed in claim 17, in which the filter is arranged topass a band of frequencies including 900 Hz.
 19. An apparatus as claimedin claim 17, in which the filter is arranged to pass a band offrequencies including 1080 Hz.